Each TimelineItem is a high level grouping of samples, representing either a Visit or a Path, depending on whether the user was stationary or travelling between places. The durations can be as brief as a few seconds and as long as days (eg if the user stays at home for several days).
Inside each TimelineItem there is a time ordered array of LocomotionSample samples. These are found in timelineItem.samples. The first sample in that array is the sample taken when the timeline item began, and the last sample marks the end of the timeline item.
LocomotionSamples typically represent between 6 seconds and 30 seconds. If location data accuracy is high, new samples will be produced about every 6 seconds. But if location data accuracy is low, samples can be produced less frequently, due to iOS updating the location less frequently.
The maximum frequency is configurable, with TimelineManager.samplesPerMinute
So for something like a Path timeline item, for example a few minutes walk between places, the Path object itself will have an activityType of .walking, but there are also all the individual samples that make up that path, some of which might not be .walking. For example if the user walks for a minute, pauses for a few seconds, then starts walking again, there might be a .stationary sample somewhere half way through the array.